If driver installation fails, you may need to disable driver signature enforcement temporarily. Go to Settings → Update & Security → Recovery → Advanced Startup → Restart now → Troubleshoot → Advanced Options → Startup Settings → Restart → Press F7 to disable signature enforcement.
The v0.8.6.0 update introduced several critical changes that streamlined the user experience:
The FTF file of the Generic Swiss update (SI: 1257-3740] has just been released. The update will upgrade your firmware build to 6. Xperia Blog How to flash your device - Sony's Developer Portal
A high-quality, high-speed data cable plugged directly into a motherboard USB port (prefer USB 2.0 ports over USB 3.0 for better driver compatibility). Step-by-Step Guide to Flashing Firmware with v0.8.60